Anthony Amore, current security director at the Isabella Stewart Gardner Museum in Boston, stands beside empty frames from which thieves took "The storm on the Sea of Galilee," center rear, by Rembrandt and "The Concert," right front, by Vermeer. AP Photo/Josh Reynolds.

BOSTON (AP).- It remains the most tantalizing art heist mystery in the world.

In the early hours of March 18, 1990, two thieves walked into Boston's elegant Isabella Stewart Gardner Museum disguised as police officers and bound and gagged two guards using handcuffs and duct tape. For the next 81 minutes, they sauntered around the ornate galleries, removing masterworks including those by Rembrandt, Vermeer, Degas and Manet, cutting some of the largest pieces from their frames.

By the time they disappeared, they would be credited with the largest art theft in history, making off with upward of a half-billion dollars in loot far too hot to sell.

Now, 20 years later, investigators are making a renewed push to recover the paintings. The FBI has resubmitted DNA samples for updated testing, the museum is publicizing its $5 million, no-questions-asked reward, and the U.S. attorney's office is offering immunity.

Two billboards on Interstates 93 and 495 are also advertising the reward.

"Our priority is to get the paintings back," U.S. Attorney Carmen Ortiz said. "If someone had information or had possession of the paintings, immunity from prosecution is negotiable."

Investigators say they've largely ruled out some of the more popular theories, from the specter of a recluse billionaire art collector to the hand of notorious Boston gangster Whitey Bulger.

More likely, investigators say, the two were homegrown thieves with knowledge of the museum's security system  including the absence of a dead man's switch that would have alerted police once the guards became incapacitated. They might have even underestimated the breathtaking scope of their crime.

"I picture the thieves waking up the next morning and looking in the papers and saying, 'We just pulled off the largest art theft in history,'" said Anthony Amore, the museum's security director.

The theft began around 1:24 a.m. when the two white men  one in his late 20s to mid-30s, the other in his early to mid-30s  overpowered the guards, according to an FBI report.

The two took their time. A full 24 minutes passed before they were first picked up on a motion detector entering the museum's second floor Dutch room, where the most valuable paintings were seized.

Investigators believe the first nabbed was Rembrandt's iconic "Storm on the Sea of Galilee," measuring about 5-by-4 feet and dating to 1633. The frame was laid on the floor where one of the thieves neatly sliced it from its frame.

Next was "Landscape with an Obelisk" by Govaert Flinck. Other stolen masterpieces included a second Rembrandt also cut from its frame, "A Lady and Gentleman in Black" from 1633.

The most valuable piece was Vermeer's "The Concert," an oil painting measuring about 2 1/2-by-2 feet from 1660  one of only 36 known works by the Dutch master and valued at more than $250 million, Amore said.

A Rembrandt self-portrait from 1629  one of the museum's most valuable paintings  was removed from the wall, but then left untouched while one of the crooks patiently unscrewed and removed from its frame a tiny Rembrandt etching slightly larger than a postage stamp.

It was the first of many odd twists investigators have puzzled over as they mapped the route the thieves using motion detector records.

After the heavier works of art were removed from the walls, the thief in charge  possibly the older of the two  might have let the other thief take what he wanted.

Amore believes the second thief found his way to a nearby gallery, lifting smaller Degas drawings of horses while passing up more valuable works of art including one by the Italian painter Botticelli.

The thieves also tried to remove a flag of Napoleon's First Regiment from its frame before giving up and making off with a bronze finial in the shape of an eagle from atop the flag  ignoring more valuable letters with Napoleon's signature.

Then came a final puzzle.

The thieves found their way to a gallery on the first floor, again passing more valuable works of arts, to seize a "Chez Tortoni," a Manet painting of a man in a top hat and a departure from the Dutch paintings  all without triggering a motion detector.

"If we ever speak to the thieves, which is secondary, I would like to say, 'Why did you take that? Why did you pass by the Raphael?'" Amore said.

On their way out, the two thieves smashed their way into the security office and snatched the only visual record of their crime  a VHS tape.

In all, 13 works disappeared.

FBI agent Geoffrey Kelly, who has led the investigation for eight years, said it's unlikely the thieves destroyed the art.

"If it were any other kind of commodity, I might feel pessimistic about recovery, but with art it's not uncommon to stay missing for long periods of time," he said. "It's one of the most interesting novels you could write, except it's missing that last chapter."

For those drawn to what happened that March night, the lure of the theft won't fade.

Ulrich Boser, author of "The Gardner Heist" and a senior fellow at the Center for American Progress, said he's convinced the paintings were taken by burglars involved in Boston's organized crime rackets in the 1980s. He said the thieves might have subsequently lost possession of the works.

"For the most part, thieves steal these works because it's easy to do and they're worth a lot of money, and then they become too hot," he said. "You can't sell them on eBay. You can't bring them into an auction house."

Amore said he won't stop until the paintings again fill the empty frames still hanging in the museum's galleries.

"I don't have any doubt we are going to recover them," he said. "There's nothing we're not doing."